1. Field of the Invention
The present invention relates to a sheet stacking apparatus and an image forming apparatus, and relates particularly to a sheet stacking apparatus, which can align a sheet stacked on a stack tray, and an image forming apparatus provided with the sheet stacking apparatus.
2. Description of the Related Art
In the prior art, there has been known a sheet stacking apparatus which aligns a sheet discharged onto a stack tray, stacking a sheet, in a sheet discharge direction and a width direction perpendicular to the sheet discharge direction and enhances a sheet taking-out property of a sheet formed with an image (see, U.S. Patent Application Publication No. 2002/0079642 A1).
For example, a sheet stacking apparatus disclosed in U.S. Patent Application Publication No. 2002/0079642 A1 is provided with a discharge direction aligning portion which aligns a sheet in a sheet discharge direction and a width direction aligning portion which aligns the sheet in a width direction perpendicular to the sheet discharge direction. After the sheet is discharged onto the stack tray, the sheet is aligned in the discharge direction and the width direction. Specifically, the discharge direction aligning portion is provided with an abutment portion provided on an upstream side and a transport member which transports the sheet discharged onto the stack tray toward the abutment portion, and the sheet is abutted against the abutment portion by the transport member to align the sheet in the discharge direction. On the other hand, the width direction aligning portion is provided with a pair of aligning members, and the pair of aligning members is operated in the width direction to be brought into contact with end surfaces of a sheet to align the sheet in the width direction. Those operations are performed for each discharge of a sheet, whereby the sheet stacking apparatus can align all sheets in the discharge direction and the width direction.
In the sheet stacking apparatus disclosed in U.S. Patent Application Publication No. 2002/0079642 A1, the transport member is disposed below a sheet discharge portion which discharges a sheet onto the stack tray. Thus, after the discharged sheet is dropped on the stack tray by its own weight, the sheet is aligned in the discharge direction by the transport member. Consequently, when thin paper tending to be more frequently used recently is used, the self-weight drop time is long, and therefore, a timing of starting alignment operation is delayed. As a result, when the sheet stacking apparatus is used in an image forming apparatus with high productivity, there is a problem that the next sheet is discharged before completing the alignment operation and the alignment processing cannot be performed. Moreover, the posture of the thin paper during the self-weight dropping is unstable, and this easily prevents the alignment operation. As described above, the above sheet stacking apparatus has a problem that it cannot correspond to the image forming apparatus with high productivity and thin paper.
In contrast, it is considered that a transport member capable of lifting and lowering is disposed above the sheet discharge portion, and the transport member is lowered for each discharge of a sheet to drop the sheet forcibly, whereby sheet dropping time can be reduced, and, at the same time, the dropping posture can be stabilized. However, when the transport member is disposed above the sheet discharge portion, a user may touch the transport member when the user takes an aligned sheet, for example. When the user touches the transport member, the transport member may be damaged, and thus it is not preferable.
Thus, this invention provides a sheet stacking apparatus, which does not lower a sheet taking-out property of a sheet discharged onto a stack tray and can correspond to thin paper and an image forming portion with high productivity, and an image forming apparatus provided with the sheet stacking apparatus.